NGK Resistor Covers are manufactured using extremely durable Phenolic Resin and EPDM Rubber. Ceramic resistors are used to ensure optimum performance and reliability. Resistor Covers must withstand extreme heat, vibration, prevent flash over and must be water proof. NGK Spark Plugs superior design ensures optimal service life and performance. Resistor Covers are for Motorcycle, Marine, Snowmobile and Small Engine applications only.